Cy 24 Phase I Monitoring the 3 Primary WD Standard Stars ------------------------------------------ Bohlin, Proffitt, Debes, Monroe (5 orbits) The three nearly pure hydrogen WDs G191B2B, GD153, and GD71 provide the basis of the HST flux calibrations for all the SIs. High fidelity models for each star are normalized with respect to the monochromatic flux of Vega at 5556A in order to establish standard candles in the sky. The flux of any star in the sky can be measured by the instrumental response relative to the ensemble response of these three fundamental standards. Repeated observations of these 3 primaries can reveal any variability in one of the 3, as well as in any other monitor star such as our MAMA and CCD monitor standards GRW+70D5824. and AGK+81D266. Historically, STIS has observed this primary WD triad with a recent cadence as frequent as all 3 stars every year. Rather than putting all the eggs in one basket and depending only on single sensitivity monitoring stars, prudence suggest that return visits in low dispersion by STIS to the fundamental triad should be executed at something approaching the historic cadence. In addition to variability checks, each new observation adds to the statistical precision of the STIS flux calibration that is defined by these 3 stars. Because about 3 years will have elapsed between these new data and the last set of STIS spectral observations, all 3 stars should be observed in Cycle 24. In subsequent years, perhaps a 3 orbit cadence of G191B2B every year plus one of the other two WDs would be sufficient. The brighter G191B2B requires only one orbit, because only the CCD can be used, while the fainter two standards require two orbits to obtain both MAMA and CCD spectra in low dispersion. The Figures illustrate the average change in response for the 5 STIS low dispersion modes in comparison to the triad of primary standards. The uncertainty in the STIS TDS correction is determined by the level of agreement between the triad of WDs and the respective monitoring standards GRW+70D5824 or AGK+81D266. Observations are in the 5 low dispersion modes G140L, G230L, G230LB, G430L, and G430LB, while the analysis will be similar to that shown in the Figures to measure any differences in the TDS between the monitor stars and the prime WDs. RA DEC Type V G191B2B 5 05 30.62 +52 49 54.0 DA0 11.781 Primary ~pure H WD GD71 5 52 27.51 +15 53 16.6 DA1 13.032 Primary pure H WD GD153 12 57 02.37 +22 01 56.0 DA1 13.346 Primary pure H WD Exposure times: Mode GD191B2B GD153 GD71 G140L …. 596 595 G230L …. 592 595 G230LB 468 656 640 G430L 220* 260* 260* G750L 1058 1974 1974 * X2 for doing both centered and E1